The invention provides a method for measuring the content of free water. The method comprises the steps of measuring the mass values of a sample at different times under the condition of constant temperature or temperature rise, wherein the temperature range of constant temperature or temperature rise is 20-50 DEG C; generating a weight loss curve according to the measured mass values, wherein the x-coordinate of the weight loss curve is time, and the y-coordinate of the weight loss curve is the mass values; acquiring a water loss rate change curve according to the weight loss curve, and acquiring the time corresponding to peak value from the water loss rate change curve; acquiring the mass value which corresponds to the time corresponding to the peak value according to the weight loss curve; working out the difference value of the mass value which corresponds to the time corresponding to the peak value and the mass value corresponding to the initial time in the weight loss curve, thereby acquiring the mass of the free water in the sample. After the method is used, the problems that the existing Abbe refractometer method is poor in data accuracy and stability during measurement and is not suitable for the sample containing the lower content of free water can be solved; furthermore, the sample is measured in time, and the detection cost is low.